Specs & measurements
Sensitivity
Also called SPL rating, dB/mW, dB/V
Sensitivity says how loud a headphone or speaker plays for a given input: so many decibels per milliwatt or per volt for headphones, per watt at one meter for speakers. Higher numbers play louder from less power.
The catch is the reference. Headphone makers quote either dB per milliwatt (dB/mW) or dB per volt (dB/V), and the two can differ by up to about 20 dB for the same product, depending on its Impedance. A 300 Ω headphone rated 97 dB/mW is about 102 dB/V; a 32 Ω headphone rated 97 dB/mW is nearer 112 dB/V. Comparing a dB/mW figure with a dB/V one tells you nothing.
Because makers rarely state which reference they use, audiophila. shows sensitivity as the listed number with a note that the reference was not recorded, and does not chart or rank it. Treat it as a hint about how hard a product is to drive, read together with impedance.
Speaker sensitivity, in dB at 1 W (or 2.83 V) and 1 m, is more consistent, though a 2.83 V rating on a 4 Ω speaker reads 3 dB higher than the same speaker rated at 1 W. An 85 dB speaker needs roughly four times the amplifier power of a 91 dB one to reach the same level, which matters more than most other specifications when pairing with an Amplifier.